1. Field of the Invention
This invention relates generally to electrochemical cells, and, more specifically, to ionic liquid gel electrolyte chemistries and methods of making batteries that can be used with devices as single-use or rechargeable power sources.
2. Description of Related Art
The reduction of electronic device form factors and their power demands have made it possible to realize new devices that are thin, compact, and lightweight. The evolution of portable devices can be in part attributed to the combination of the advancements in battery electrode materials and their compatibility with electrolyte materials. For example, the development of more effective high energy density lithium and lithium-ion electrode materials has enabled portable, compact, high capacity batteries, while the introduction of lithium and lithium-ion solid polymer and gel electrolytes has relaxed the battery's requirement for rigid and hard packaging, spurring the wide-spread adoption of thinner batteries, hermetically sealed within pouch material. In addition to performance and form factor benefits, the use of solid-state, polymer, and gel electrolytes have introduced additional improvements in battery manufacturability, cost, and inherent safety. Thus, considerable efforts have been dedicated to solid-state, polymer, and gel electrolyte development.
The demand for thin, miniature, and low cost batteries has been propelled by the increased ubiquity of low power sensors, wireless devices, and printed electronics. The reduction of electronic device form factors and their power demands have made it possible to realize a fully-integrated microdevice platform, with computation, communication, and sensing capabilities all enabled by an integrated power source. In particular, MEMS-based sensors and actuators, especially in autonomous, integrated platforms known as “smart dust” have been huge drivers in the development of thin format battery and microbattery technology. The implications of the widespread deployment of these devices, especially autonomous wireless sensor nodes, is pivotal to a variety of fields including the “internet of things”, wearable electronics to enable the “quantified self”, smart labels, intelligent toys, structural monitoring, and cost- and energy-effective regulation of home, industry, and office energy use applications, to name a few. These broad classes of devices require power sources that can supply power in the range of microwatts (μW) to hundreds of milliwatts (mW), and capacities from microamp-hours (μAh) to hundreds of milliamp-hours (mAh), depending on the application. In addition, for many of the portable or ubiquitous applications, it is desired that the power source is no greater in size than the device it powers, and thin in form factor. Finally, low cost and mass-manufacturable solutions are critical.
Of the existing battery systems that are being considered for these applications, thin film, lithium polymer, and semi-printed batteries are the forerunners, though each have significant shortcomings that have limited their widespread adoption. Vapor deposited thin film lithium and lithium-ion batteries have low storage capacities and power capabilities due to materials deposition limitations. Lithium polymer batteries have leveraged the rapid advancements of pouch cell battery manufacturing, but like thin film lithium and lithium-ion batteries, are plagued by stringent hermetic encapsulation requirements due to its sensitivity to contamination from the environment. Semi-printed batteries often utilize a liquid electrolyte, adding cell geometry and manufacturing complexities.
Such microdevices need power sources with footprints less than 1 cm2 and thicknesses on the order of a few mm or less, that can supply power in the range of microwatts (μW) to milliwatts (mW), depending on the application. The need for a micropower source that can satisfy the power requirements of such wireless devices and with comparable dimensions has incited a surge of research within the fields of microfabrication, energy harvesting, and energy storage. For autonomous wireless sensors, the microenergy storage devices currently being considered are microbatteries and microcapacitors.
Although microbattery chemistries may be similar to macrobattery chemistries, macrobattery configurations, packaging, and post-processing are not feasible below the centimeter scale. As a result, in addition to materials optimization, microbattery researchers have focused heavily on integrating microbatteries directly onto the same substrates as the devices they are powering.
An ideal microbattery (or microcapacitor) solution has not yet been found. Nickel-zinc systems have the problem that zinc dendrites grow, and the shape of the electrode changes during cycling, thus reducing cycle life. Rechargeable alkaline manganese cells and zinc-silver oxide cells have the same problem. Lithium-ion and lithium polymer systems require strict charge and discharge regulation and pose flammability risks.
What is needed is a safe, long-lasting, inexpensive micropower source that can enable microdevices to be used in a wide variety of applications.